The proposed research seeks (1) to explore the development of auditory perceptual abilities, specifically pitch and timbre perception, during infancy, and (2) to delineate the acoustic cues infants use in those tasks as a means of determining the mechanisms underlying complex auditory processing. Both pitch and timbre perception appear to require the integration or synthesis of information gained from a peripheral analysis on the spectral characteristics of signals. Whereas it is known that infants can synthesize pitch and timbre from clear, unambiguous acoustic signals, and understanding of the mechanisms underlying those perceptions requires that the range of stimuli presented to infants be expanded and that the limits of perception be determined. The studies of pitch perception will assess the limits of infants' abilities to synthesize pitch from a variety of complex spectra that have been used with adults. Specifically, these studies will (1) assess the synthesis of pitch from harmonic tonal complexes containing different numbers of spectral components by 7-month-old infants and naive adults; (2) test infants' extraction of pitch from inharmonic tonal complexes with varying degrees of inharmonicity; (3) evaluate infants' perception of the missing fundamental from signals having high fundamental frequencies, which do not elicit a pitch for adults; and (4) measure the effects of different-frequency maskers on infants discrimination of pure tone and tonal complexes. Studies of timbre perception will move beyond the simple discrimination already demonstrated and attempt to determine the basis of infants' responding. Specifically, these studies, will (1) test infants' discriminate; and (2) assess infants' abilities to integrate spectral information across widely separate frequency regions in a profile analysis task. Seven-month-old infants will be tested in a conditioned head turning procedure where thy will be asked to discriminate complex acoustic signals and to categorize those signals on the basis of pitch or timbre. By delineating the nature of complex auditory perception for infants, the proposed research will provide an important first step in the formulation of a general model of auditory development. Comparisons of infants' performance with that of adults will indicate the degree to which adult theories of auditory perception can be generalized to infants and highlight the magnitude and quality of the developmental changes that must be accounted for by models of auditory development.